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The strike took place Saturday, 120 miles north of Mogadishu at a training site

A U.S. official told CNN that the Al-Shabaab fighters were standing outside in military formation when the strike occurred

Washington (CNN)In a move to stop what the U.S. military said was an imminent threat against U.S. troops and African peacekeeping forces in Somalia, a U.S. strike in Somalia killed as many as 150 suspected Al-Shabaab fighters, the Pentagon said Monday.

The Pentagon initially characterized the attack as a drone strike, but later clarified that the operation involved both manned and unmanned aircraft.

The strike took place Saturday, 120 miles north of Mogadishu at a training site called "Raso Camp," according to Pentagon spokesman Capt. Jeff Davis.

The camp had been under observation by U.S. Special Operations forces for several weeks, Davis said. The camp had about 200 fighters on site including Al-Shabaab trainers.

Separately, a U.S. official told CNN that the Al-Shabaab fighters were standing outside in military formation when the strike occurred.

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Intelligence indicated the group was training for some time and was in the final stages of getting ready to conduct a "large-scale attack," Davis said.

Pentagon spokesman Peter Cook said in a statement that the attack was taken in defense of "our African Union Mission" in Somalia and that "The fighters who were scheduled to depart the camp posed an imminent threat" to the mission.

He added, "The removal of these fighters degrades al-Shabaab's ability to meet the group's objectives in Somalia, including recruiting new members, establishing bases, and planning attacks" on U.S. and African Union forces.

The military did not specify the location of where the U.S. thought an attack might happen, but it is well known that the U.S. maintains a limited military presence at the airport in Mogadishu and that small numbers of U.S. Special Operations forces have traveled to other locations in the country.